Ecological requirements tend to control the order in which different species arrive
Example: Eastern White Pine is able to survive on poor, dry soil, with no shade. Hemlock requires a richer moister soil and shade (Raven and Johnson, 1992). As a result, the migration of the two species did not occur simultaneously. Hemlock did not start north until approximately 1000 years after White Pine due to its specific ecological requirements (E.C. Pielou, 1991).
Example: Evidence shows that White Spruce consistently arrived first into an area relative to Black Spruce. The White Spruce is able to grow on inorganic, newly uncovered soil. Black Spruce must lag behind as it requires spongy organic soil where moss has accumulated (J.D. Sauer, 1988).